The present invention relates to the field of Group IV compounds. In particular, this invention relates to the preparation of Group IV organometallic compounds suitable for use in chemical vapor deposition.
Metal films may be deposited on surfaces, such as non-conductive surfaces, by a variety of means such as chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), and other epitaxial techniques such as liquid phase epitaxy (“LPE”), molecular beam epitaxy (“MBE”), chemical beam epitaxy (“CBE”) and atomic layer deposition (“ALD”). Chemical vapor deposition processes, such as metalorganic chemical vapor deposition (“MOCVD”), deposit a metal layer by decomposing organometallic precursor compounds at elevated temperatures, i.e., above room temperature, either atmospheric pressure or at reduced pressures. A wide variety of metals may be deposited using such CVD or MOCVD processes.
For semiconductor and electronic device applications, these organometallic precursor compounds must be highly pure and be substantially free of detectable levels of both metallic impurities, such as silicon and zinc, as well as oxygenated impurities. Oxygenated impurities are typically present from the solvents used to prepare such organometallic compounds, and are also present from other adventitious sources of moisture or oxygen.
For certain applications where high speed and frequency response of an electronic device is desired, silicon-only devices, e.g. silicon bipolar transistors, have not been competitive. In a heterojunction bipolar transistor (“HBT”), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. The speed and frequency response of a silicon-germanium HBT are comparable to more expensive gallium-arsenide HBTs.
The higher gain, speeds, and frequency response of silicon-germanium HBTs have been achieved as a result of certain advantages of silicon-germanium not available with pure silicon, for example, narrower band gap and reduced resistivity. Silicon-germanium may be epitaxially grown on a silicon substrate using conventional silicon processing and tools. This technique allows one to engineer device properties such as the energy band structure and carrier mobility. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field or potential gradient, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. A common method for fabricating silicon and silicon-germanium devices is by CVD. A reduced pressure chemical vapor deposition technique (“RPCVD”) used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer as well as precise control over the doping profile.
Germane (GeH4) is the conventional precursor for germanium deposition while silane (SiH4), and dichlorosilane (SiH2Cl2) are conventional precursors for silicon deposition. These precursors are difficult to handle and have high vapor pressures. For example, germane decomposes violently at 280° C., which is below the temperature used to grow germanium films. Accordingly, processes employing either germane or silane require extensive safety procedures and equipment. Germane typically requires film growth temperatures of approximately 500° C. or higher for thermal CVD applications. Such decomposition temperatures are not always suitable, such as in mass production applications where there is a need for lower temperatures, e.g. 200° C. Other CVD applications require higher growth temperatures, which cause conventional precursors to break up prematurely which, in turn, leads to the formation of particles and a reduction in metal film growth rates. A further problem with conventional silicon and germanium precursors is that when a relatively stable silicon precursor and a relatively unstable germanium precursor are used to deposit a silicon-germanium film, the differences in precursor stability make control of the silicon-germanium composition difficult.
There is a need for precursors for silicon and germanium vapor phase deposition that are safer to handle and have decomposition temperatures that are tailored to specific conditions. Certain silicon and germanium precursors having desirable properties for use as CVD precursors include the organosilicon hydrides, organogermanium hydrides and heteroleptic organosilicon and organogermanium compounds. Such Group IV organometal precursors may be difficult to prepare and may involve multiple steps. For example, the use of trialkylaluminum compounds for the alkylation of Group IVA metals (e.g., silicon, germanium and tin) and Group VIA metals (e.g., selenium and tellurium) has not been successful because of certain problems encountered. For example, the reactions between aluminum alkyls and germanium halides are known to produce di- and poly-germanes rather than desired alkylgermanes as final products. Similarly, the reaction between tellurium halides and organoaluminums is known to form aluminum telluride as the final product rather than the desired dialkyltellurides.
Accordingly, the reactions employed for the synthesis of Group IVA and Group VIA organometallics are primarily based on organolithium and organomagnesium compounds. These reactions inherently involve the use of ethereal solvents that are extremely difficult to remove at ppm levels. Also, organotellurides are known to be commercially synthesized in high yields by using aqueous medium and in the presence of a phase transfer catalyst. See, for example, U.S. Pat. No. 5,442,112. Such processes involve oxygenated solvents and thus create serious quality concerns for the use of these products in certain electronics applications, where trace oxygen and organics are known to catastrophically affect the optoelectronic properties of the fabricated devices.
Accordingly, there is a need for a method of preparing organometallic compounds, such as Group IVA and Group VIA alkylmetal compounds, for use as CVD precursors where such method involves fewer steps than conventional methods, and where such compounds are substantially oxygen-free.